It is known that 1,4-diaminoanthraquinone-2-sulfonic acid is obtained by reacting 1-amino-4-bromoanthraquinone-2-sulfonic acid or a salt thereof with liquid ammonia in the presence of a copper catalyst (see German Pat. Nos. 1,142,174 and 1,155,786). In this method, however, the desired product deposits on reactor walls, giving rise to problems such as deterioration of the product due to local overheating, a reduction in heat transfer efficiency particularly at the recovery of ammonia, and deterioration in the handling properties at the post-treatment step. Thus, this method would not be suitable for the production of 1,4-diaminoanthraquinone-2-sulfonic acid or its salt at least on an industrial scale.
In order to overcome the above-described problems, there have been proposed a method in which the reaction is carried out in the presence of formamide or N-methylformamide (see Japanese Patent Application (OPI) No. 123960/81 (the term "OPI" as used herein means a "published unexamined Japanese patent application")), and a method in which ammonia water is used in place of liquid ammonia (see, for example, Japanese Patent Application (OPI) No. 108059/82). These methods, however, suffer from disadvantages in that the yield and purity of the desired product are seriously low as compared with those by the reaction being carried out in liquid ammonia.